3 3/8" is the way to go. You modify the gauge cup to fit the dash and you end up with the gauge taking up the entire space in the original hole, so you get a full size gauge and not some doubled up bezel with a little gauge in the middle.
I used this one
Stewart Warner Heavy-Duty Series Tachometers 82171
I had to cut the gauge cup down some so it would be spaced back correctly and fit through the back of the gauge. Then I cut apart the original cup that was in the rallye dash to block the tach and epoxied it to the new gauge cup. End result bolts in just like a stock tach. It will protrude further back, but it should clear everything.
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I still have to repaint the needle but that's easy enough. It'll look better in my actual dash, the one in the pictures is a parts dash and the lenses are kinda foggy.
The problem with that is the 3 3/8" gauges have a larger face than the original tachs did. I think you'd have a hard time matching up the sweep of the new tach to match the old face, the calibration would be hard to get right and if you didn't the tach wouldn't be accurate.
